The OSNAP array is an observing system between Scotland and Labrador, Canada which provides continuous measurements of heat and freshwater transport in the North Atlantic Subpolar gyre. These observations allow scientists to track the strength of the Atlantic Meridional Overturning Circulation (AMOC) which exerts a major influence on Northern European and Arctic climate. UK-OSNAP maintains seven long-term moorings, which along with annual CTD stations and glider occupations capture the major current pathways in the Northeastern Atlantic.  I am using data from the OSNAP array to investigate the long-term controls and changes in circulation at the eastern and western margins of the Rockall Trough.

Rockall Bank is an isolated topographic feature approximately 350 km west of the Outer Hebrides in North-west Scotland. The bank is flanked on its western side by the Hatton- Rockall Basin, and on its eastern side by the Rockall Trough.

West of Rockall Bank, a filament of the North Atlantic Current comprised of warm, salty North Atlantic Water (the ‘Rockall Bank Jet’) flows northward with a mean transport of 1.6 Sv (Houpert et al., 2018). To the east of the bank, net flow is persistently southward (Holliday et al, 2018) with a mean transport of 1.9 Sv. The structure of this flow is nearly barotropic, and we see the highest velocities near the seabed.

There is strong evidence in the form of current meter data, hydrographic sections and robotic glider occupations that these currents connect at the north and south of the bank, forming a closed circulation. This raises the possibility of Rockall Bank system resembling a ‘Taylor Column’ (Dooley, 1984, Taylor, 1922); a vertical column of fluid constrained to move parallel to the sloping bathymetry of the bank.

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Glider depth-averaged current observations from multiple missions binned into a transect crossing northern Rockall Bank, after Houpert et al. (2018). Green points show glider profile locations.
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The location and average current velocity measured by the OSNAP WB1 mooring.

 

The OSNAP western basin (WB) mooring time series is the longest measure of full-depth temporal variability within the Rockall Bank current system. At present this mooring provides 4 years of high resolution T, S and current data in the southward flowing portion of the circulation. Changes in current speed often impact the full depth of the water column, however periods of northward flow sometimes extend from the surface to 1000 m. A wavelet frequency analysis of the current core shows that significant fluctuation of current speed occurs mainly at tidal (semi-diurnal) and 5 month periods, with some variability also present over periods of 1 – 2 months in summer 2017.

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Current speed at mooring WB1 (north +ve). a) depth-mean current, b) 2D contour (30 day low pass filtered), c) time-mean current. Star shows depth of instrument analysed in the next figure.
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Wavelet frequency analysis of current meter data at 1350 m at WB1 for the period July 2016 to July 2018. Periods where variance is significant at 95 % confidence above a red noise background are contoured in black. Greyed-out region delineates ‘cone of influence’ where edge effects reduce data confidence. For more information on method see Torrence and Compo (1998).

If Rockall Bank is largely isolated from surrounding waters by bathymetrically controlled currents, this limits the cross–fertilisation of larvae between the bank and the neighbouring ocean. For a fish population to be indigenous to the bank for example, there must be a limited exchange of waters during the egg and larval stages of the species. The outlook for that species would then be influenced by inter-annual fluctuations in ocean-bank exchange.

We aim to develop a quantitative physical framework to explain the behaviour of the Rockall Bank circulation and its sensitivity to external forcing. While local wind does not appear to influence the current, it may instead to respond to basin-scale integrated wind forcing (the ‘island effect’) as has been demonstrated for the circulation around Hawaii (Qiu et al., 1997). Further investigation is also required to understand the relationship between current speed and tidal energy.

Once a physical framework exists, we hope to apply the findings to biological systems using empirical flux calculations or modelled particle tracking experiments.

References

Dooley, H. D. 1984. Aspects of oceanographic variability on Scottish fishing grounds. Ph.D. Thesis, University of Aberdeen.
Holliday, N. P., S. Bacon, S. A. Cunningham, S. F. Gary, Johannes Karstensen, B. A. King, F. Li, and E. L. Mcdonagh, 2018 “Subpolar North Atlantic overturning and gyre‐scale circulation in the summers of 2014 and 2016.” Journal of Geophysical Research: Oceans 123, no. 7: 4538-4559.
Houpert L., M. Inall E. Dumont S. Gary C. Johnson M. Porter W. E. Johns S. A. Cunningham, 2018. Structure and Transport of the North Atlantic Current in the Eastern Subpolar Gyre from Sustained Glider Observations, Journal of Geophysical Research-Oceans, https://doi.org/10.1029/2018JC014162
Taylor, G.I., 1922. The motion of a sphere in a rotating liquid. Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 102(715), pp.180-189.
Torrence, C. and Compo, G.P., 1998. A practical guide to wavelet analysis. Bulletin of the American Meteorological society, 79(1), pp.61-78.
Qiu, B., Koh, D.A., Lumpkin, C. and Flament, P., 1997. Existence and formation mechanism of the North Hawaiian Ridge Current. Journal of Physical Oceanography, 27(3), pp.431-444.